Spin-valves based on the effect of giant magnetoresistance and tunneling magnetoresistance are currently used in high-density magnetic recording heads and magnetoresistive random-access memories. This type of device is based on electrical resistance having two different values; say R1 and R2 that are dependent on an applied external magnetic field. When a magnetic head is in proximity to the spin-valve device it can change the resistance, or it can change voltage if an electric current runs through the device, between the two resistance values R1 and R2. The change in electrical resistance does not involve extra current or voltage; it just reacts to the external magnetic field. A spin-valve can be regarded as a switch, wherein the application of an external magnetic field does the switching.
A conventional vertical spin-valve device can be constructed using two thin ferromagnetic layers (each with a thickness of less than 100 nm) and a spacer in between, which can be a metallic or insulating thin layer (a few nm thick). When the magnetization orientation in the two adjacent ferromagnetic electrodes is parallel to each other, the electrical resistance measured perpendicular to the films has value R1; alternatively, when the two magnetization orientations of the two ferromagnetic films are anti-parallel to each other then the resistance is R2, which is different than R1. The magnetization of the electrodes can be arranged to be parallel or anti-parallel to each other by an external magnetic field. The resistance change under the influence of the magnetic field has been dubbed magnetoresistance or MR.